Since 1978, satellites have made continuous observations of Arctic sea ice. In that time, sensors have found an overall decline in its extent. Beginning in 2002, this decline steepened, with early onset of springtime melt north of Siberia and Alaska. Beyond summertime melt, Arctic sea ice further surprised researchers in the winter of 2004-2005. “Even if sea ice retreated a lot one summer, it would make a comeback the following winter, when temperatures fall well below freezing,” explains Florence Fetterer of the National Snow and Ice Data Center (NSIDC). “But in the winter of 2004-2005, sea ice didn't approach the previous wintertime level.” With the exception of May 2005, every month between December 2004 and September 2005 saw the lowest monthly average since the satellite record began.

This graph shows the five-day mean sea ice extent for July through September for the years 2002 through 2005. All four years were below the average sea ice extent for 1979-2000 (gray line). In fact, recent sea ice extent falls below the 1979-2000 average by an area twice the size of Texas. On September 19, 2005 (the latest date shown on this graph), Arctic sea ice extent fell to 5.35 million square kilometers (2.06 million square miles). It continued to decline until September 21, 2005, when it dropped to 5.32 million square kilometers (2.05 million square miles). This new low was 670,000 square kilometers (approximately 258,000 square miles) below the previous record low in 2002.

From 1979 through 2001, the rate of September Arctic sea ice decline was just over 6.5 percent per decade. The September 2002 minimum increased this rate to 7.3 percent. Incorporating the sea ice extent projection for 2005 increased the rate to approximately 8 percent per decade.

Patterns of natural variability play a part in Arctic sea ice decline. The Arctic Oscillation is a major atmospheric circulation pattern that can take a positive or negative mode. In its positive mode, it sets up winds that tend to break up sea ice and flush it out of the Arctic, and the thin ice left behind is more likely to melt. In its strongly positive phase in the early to mid-1990s, the oscillation may have made sea ice more vulnerable to summertime melt. Since the late 1990s, however, the Arctic Oscillation has exhibited a more neutral mode, while sea ice has continued to decline. Sea ice decline has persisted through different patterns of precipitation, wind, and local temperature variation. Researchers have found marked declines in sea ice difficult to explain without considering overall Arctic warming.

Sea ice decline is likely to affect future temperatures in the region. Because it is white or light in color, sea ice reflects much of the Sun’s radiation back into space, whereas dark ocean water absorbs more of the Sun’s energy. As sea ice melts, more exposed ocean water changes the Earth’s albedo, or fraction of energy reflected away from the planet. The increased absorption of energy further warms the planet. “Feedbacks in the system are starting to take hold,” says NSIDC’s lead scientist Ted Scambos. “There doesn't appear to be a way to turn this around, or even slow it down,” in a warming climate. Claire Parkinson, senior scientist of NASA Goddard Space Flight Center points out a potential mitigating factor, noting that “the reduced sea ice coverage will lead to more wintertime heat loss from the ocean to the atmosphere, and perhaps, therefore, to colder water temperatures and further ice growth.”

Still, recent trends caused concern. Walt Meier of NSIDC remarks, “Having four years in a row with such low ice extents has never been seen before in the satellite record. It clearly indicates a downward trend, not just a short-term anomaly.”